1. Field of the Invention
The present invention relates generally to mosaic diamond drill bit cutters of the type incorporating polycrystalline and thermally stable diamond products and more particularly to such a cutter which forms a nonuniform wear pattern during drilling. In another aspect, the invention relates to drill bits incorporating cutters which wear at different rates.
2. Description of the Related Art
One type of cutter for an earth-boring rotary drag bit is made from a plurality of polycrystalline diamond (PCD) cutting elements. The PCD cutting elements are embedded in a metal matrix having a planar cutting face. Each of the PCD elements has a planar end surface which is coplanar with the cutting face. The cutting face therefore comprises both matrix material and PCD material. During drilling, cutting occurs along a cutting edge defined by one side of the cutting face. The cutting edge is embedded partly into the rock formation and is advanced therethrough by bit rotation. During drilling, the matrix and the PCD elements therein gradually wear from the cutting edge into the matrix.
One such prior art cutter is disclosed in U.S. Pat. No. 4,726,718 to Meskin et al. for a multi-component cutting element using triangular, rectangular and higher order polyhedral-shaped polycrystalline diamond disks. The Meskin et al. cutter includes triangular PCD elements embedded in a metal matrix having a diamond grit dispersed therein.
U.S. Pat. No. 4,592,433 to Dennis discloses a cutting blank with diamond strips in grooves. In Dennis, PCD material in different shapes, including strips and chevrons, has a planar surface exposed on the cutting surface of a cutting blank. The metal cutting blank in which the PCD elements are embedded produces an irregular cutting edge as the cutting blank does not cut the formation but wears away at a much faster rate than the PCD cutting elements. U.S. Pat. No. 4,255,165 to Dennis et al. discloses a composite compact of interleaved polycrystalline particles and cemented carbide masses in which cemented carbide is interleaved with PCD material. During cutting the carbide rapidly wears away leaving the PCD cutting elements exposed in a so-called bear claw configuration in which the PCD cutting elements form spaced cutting fingers. The prior art cutters present a jagged or irregular cutting edge which in some circumstances cuts more effectively than a smooth or uniform cutting edge.
As used herein, the term wear ratio refers to the volume of a cutting element worn away relative to the volume of rock worn away during an abrasive cutting test. Such cutting tests are known in the art to which the present invention relates and involve abrading the surface of a preselected rock with a cutting element of interest. For PCD or thermally stable diamond products, the wear ratio is a function of several parameters, including diamond feedstock size, degree and type of sintering, force applied, grain size, cementation of rock and temperature. As used herein, the term wear rate refers to the rate at which a cutting element wears during drilling. The wear rate is a function of the wear ratio of the wear rate and geometry of the cutting element. Thus, cutting elements having the same wear ratio but different geometries wear at different rates. Similarly, cutting elements with the same geometry but with different wear ratios also wear at different rates.
Although the prior art PCD cutters described above produce irregular patterns on a cutting edge during wear, none incorporates a cutting edge which wears at different rates along the edge. Prior art cutters include irregularly shaped PCD material embedded in a matrix; however, the PCD elements which form the cutting edge have a uniform wear rate. While some of the prior art patents include PCD material alternating with carbide along a cutting edge, the carbide does not cut but rather simply wears away thereby leaving an irregularly shaped cutting edge but still with cutting elements all of which have a uniform wear rate. It would be desirable to provide a cutter having a cutting edge which includes cutting elements that wear at different rates to present an irregular cutting edge.
None of the prior art cutters wear at different rates. It would be desirable to have such a cutter to permit cutting with elements having a first wear rate through an initial formation having one hardness and thereafter boring through a lower formation through which it would be desirable to cut with a cutter having a different wear rate. Because the prior art cutters are made of PCD cutting elements having only a single wear rate, the wear rate of the cutting elements remains the same while the hardness of the formation through which the bit is drilling may vary. It would be desireable to provide a drill bit with cutters having a wear rate which varies in a preselected fashion to optimize cutting through formations of varying hardness.
It would also be desireable to provide a cutter which presents an increased surface area of PCD cutting elements toward the bottom of the bore hole thereby slowing wear rate of the cutting edge.
It would also be desireable to provide the same advantages as described above in connection with a rotary drag bit in a percussive drill bit.
It would be desirable also to implement such a cutter which is mounted in any fashion including bits of the type in which the cutters are integrally formed with the bit body as well as on bits of the type having stud-mounted cutters or cutters brazed to the bit body.
As discussed above, none of the prior art discloses a cutter for a rotating drag bit having PCD cutting elements which wear at different rates. Moreover, none of the prior art discloses a rotating drag bit having cutters formed of diamond cutting elements in which the cutting elements on one cutter wear at a different rate from the cutting elements on another cutter. It would be desireable to provide such a rotating drag bit in which, e.g., the cutters arranged in one blade on the bit include diamond elements having a first wear rate while cutters in another blade on the bit have a different wear rate. Such a drill bit would permit concentration of cutting action on only a few blades having a relatively low wear rate while additional blades, having a relatively high wear rate, stabilize the bit during drilling.